The present invention relates to a process for the condensation of linear poly(alkylene terephthalate) having a viscosity number of at least 50 cm.sup.3 /g in the solid phase at elevated temperature.
Poly(alkylene terephthalate), especially poly(ethylene terephthalate) (PETP) and poly(butylene terephthalate) (PBTP) are excellent materials for the manufacture of films, sheets, molded components, etc. The poly(alkylene terephthalate) generally must exhibit a viscosity number &gt;100 cm.sup.3 /g, especially when producing molded components by the injection molding and extrusion methods.
Poly(alkylene terephthalates) are customarily prepared by transesterification or esterification and subsequent polycondensation of terephthalic acid or their polyester-forming derivatives and an alkanediol in the presence of catalysts [See, e.g. Sorensen and Campbell, Preparative Methods of Polymer Chemistry, Interscience Publishers, Inc., NY (1961), pp. 111-127; "Kunstsoff-Handbuch" [Plastics Manual], vol. VIII, C. Hanser Publishers, Munich (1973), all of whose disclosures are incorporated by reference herein].
When using these processes, conducted in the melt, viscosities of the magnitude required, for example, for injection molding and extrusion purposes, cannot be reached since degradation reactions occur as early as in the polycondensation stage due to the temperature load. The desired high viscosity cannot be attained.
Another limitation on the viscosity of the polyesters in the melt condensation process is a result of the viscosity of the melt. This increases with progressive polycondensation, making it difficult and/or impossible for the volatile, low-molecular weight reaction products to escape, and thereby impeding the progress of the polycondensation. Furthermore, difficulties are encountered technically in discharging the highly viscous melt from the reaction vessel (DOS No. 2,117,748).
Insofar as necessary, a solid-phase post condensation (recondensation) is usually conducted to alleviate the problems (British Pat. No. 1,066,162; U.S. Pat. No. 3,405,098). This is done by bringing the polyester, in granulated or powder form, to temperatures lying approximately 10.degree.-60.degree. C. below the melting point. In this temperature range, the poly(alkylene terephthalate) is treated in the inert gas stream or under a vacuum until the desired high viscosity has been reached. See, e.g., U.S. Application Ser. No. 219,133, now U.S. Pat. No. 4,387,213 granted June 7, 1983 and U.S. Pat. No. 4,360,661, both commonly assigned for processes involving such a treatment in an inert gas stream in the presence of alkanediols in two steps.
The post condensation, or recondensation, can take place discontinuously, for example in a tumbler dryer (DOS No. 2,117,748) or continuously, for example in a fluidized bed process (DOS No. 1,804,551) or in a screw-type-reactor (DOS No. 2,162,618).
A basic disadvantage of the solid-phase recondensation resides in the long reaction period at an elevated temperature. As a result, although a poly(alkylene terephthalate) of the desired viscosity is obtained, the product is already damaged to such an extent that it is strongly degraded when being processed from the melt. Molded components made of such a material are brittle and prone to breakage.
Therefore, it has been proposed to use poly(alkylene terephthalates) in the ground form in order to shorten the recondensation time; however, this process is uneconomical. In addition, there is the danger of caking of the powder (DOS Nos. 2,152,245 and 2,117,748).
In another process the polyester is reacted after the recondensation with monofunctional glycidyl ethers or carbonates to improve the stability of the polyester (DOS No. 1,929,149). This method is time consuming and requires the use of compounds foreign to the polyester.